1. Field of Invention
The present invention relates to a TFT-LCD (Thin Film Transistor—Liquid Crystal Display) driving method. More particularly, the present invention relates to a TFT-LCD driving method utilizing an over-drive technique.
2. Description of Related Art
When an appropriate gray level voltage is applied to a pixel in a TFT LCD panel, the angle of liquid crystal molecule in the pixel will change correspondingly. This angle change further alters transmittance of the TFT-LCD panel so a desired gray level can be achieved. However, due to the intrinsic property of liquid crystal molecule, if the gray level has to change dramatically during two successive refresh periods, the desired angle change may not be achieved in one refresh period. This results in a blurred display, and the situation is particularly bad for a motion picture display.
One solution to this problem usually employs an over-drive technique.
The over-drive technique applies a gray level voltage higher than originally required, so the changing rate of the gray level can also be increased. FIG. 1 is a diagram illustrating the relation between the gray level and time when the over-drive technique is employed. Without employing the over-drive technique, it takes a period T1 for the pixel to change from an initial gray level Lx to a target gray level Ly. The period T1 is longer than a refresh period T0, which means the pixel can't change from the initial gray level Lx to the target gray level Ly in one refresh period. This results in a blurred display. However, by employing the over-drive technique, an over-drive gray level voltage Vy′ is applied to the pixel while the pixel needs to change from the initial gray level Lx to the target gray level Ly. Since the over-drive gray level voltage Vy′ is higher than the target gray level voltage Vy of the target gray level Ly, the angle change of liquid crystal molecule can be speeded. The desired angle change can therefore be achieved, and the pixel can display the target gray level Ly. That is, by providing an over-drive gray level voltage Vy′ higher than the target gray level voltage Vy, the changing rate from the initial gray level Lx to the target gray level Ly can be increased. This enables the pixel to change more rapidly from the initial gray level Lx to the target gray level Ly.
The relation between the initial gray level voltage, the target gray level voltage, and the over-drive gray level voltage, can be obtained from a Look-Up Table. Look-Up Table is a table providing the corresponding over-drive gray level voltage when the pixel has to change from an initial gray level voltage to a target gray level voltage. FIG. 2 shows a Look-Up Table of an 8-bits driving system. The horizontal axis represents the initial gray level voltage, and the vertical axis represents the target gray level voltage. The intersection is the over-drive gray level voltage applied to the pixel. For example, if the initial gray level voltage is V32, and the target gray level voltage is V64, the over-drive gray level voltage applied to the pixel would be V80.
FIG. 3 is a block diagram showing a TFT-LCD driving system utilizing the over-drive technique. Timing controller 30 retrieves Gn frame image data from an image data source, and retrieves a previous Gn-1 frame image data from a frame buffer 32. Timing controller 30 then compares the Gn and Gn-1 frame image data and addresses the pixels that need to be updated. Subsequently, timing controller 30 retrieves the Look-Up Table 34 stored in a memory, and converts the image data in the updated pixels to a corresponding over-drive gray level voltage. The over-drive gray level voltage is then applied to the pixel via a source driver.
However, the TFT-LCD driving system utilizing the over-drive technique still has some drawbacks. First, only the pixels where image data has to change during the two successive refresh periods will be updated. This requires several frame buffers to store the previous frame image data in order to compare the image data in the same pixel during the two successive refresh periods. However, frame buffers are expensive and dramatically increase the TFT-LCD manufacture cost. Besides, the Look-Up Table utilized in the over-drive technique is usually stored in EEPROM (Electrically Erasable Programmable Read-Only Memory). If the bits of the driving system were increased, the corresponding Look-Up Table would expand as well, and the memory capacity would also have to increase. This would further raise the manufacturing cost.